Pistons for internal combustion engines such as heavy duty compression ignition engines are complex products subject to a wide range of duty requirements. Such pistons operate in extremely harsh environments whilst having to produce good performance, emissions and durability characteristics over a wide range of loads, speeds, ambient conditions and with fuels the specification of which can vary dramatically. In a field where so many variables have significant impact on development, numerous piston designs have seen the light of day, as rarely will an off-the-shelf piston be suitable for a new or modified engine system. Furthermore, the piston can be designed from a very extensive set of design variables whereby a suitable combination of variables goes far beyond simple design selection but requires significant inventive efforts, research and development. Hence, not withstanding the prior art field for pistons already being very extensive, improvements can still be made, as new or modified engine systems tend to pose unique problem sets, requiring unique solutions.
One piston known in the prior art is an articulated piston, i.e. the upper portion (crown) and the lower portion (skirt) are only held together via the wrist pin, whereby the crown may be a steel component and the skirt may be made from aluminium. One such piston, for example a Ferrotherm® piston from Mahle® includes four axial drain passages between the oil ring groove and a lower open space. The four axial drain holes are distributed uniformly around the circumference of the oil ring groove, i.e. all drain passages are spaced 90 degrees apart from each other, with two passages being positioned over the central axis of the wrist pin bores and two passages being positioned centrally over the skirt walls. The four holes allows substantially uniform drainage of oil from all around the oil ring groove thereby lubricating the skirt walls.
Another piston known in the art is a single piece piston having only two axial drain passages arranged at 180 degrees angles relative to each other, they being located over the central axis of the wrist pin bores. One example of this piston is the Monotherm® piston manufactured by Mahle®.
Both the articulated and single piece pistons have been successful in many applications, however, it was found that in some applications these pistons may be unsuitable because of a lack of durability, excessive carbon built-up on the crown, lack of performance and excessive oil consumption and manufacturability issues.
Hence it is an object of the current disclosure to provide an improved piston design addressing at least some of the aforementioned disadvantages.